FIG. 1 is a schematic showing a perspective cross sectional view of a conventional silicon microphone 100. In some conventional silicon microphone micro-electromechanical system (MEMS) chips 100, the active areas include a very thin membrane 102, typically having a thickness of a few hundred nanometers as well as a counter electrode 104 suspended over a through hole 106. The micro-electromechanical system (MEMS) chip 100 with the membrane 102 is etched from the backside. The counter electrode 104 is also typically very thin. Both the membrane 102 and the counter electrode 104 are partially metalized. Acoustic waves will impinge on the membrane 102. This will cause the membrane 102 to oscillate. The acoustic waves are detected by measuring the capacitance change due to the oscillation of the membrane 102. The performance of the microphone usually depends on the volume on the back side of the membrane, i.e. the side opposite the front side in which acoustic wave impinges on.
FIG. 2 is a diagram showing various components that may be present in a conventional silicon microphone 200. The silicon microphone includes a micro-electromechanical system (MEMS) chip 202 with a membrane 204. The MEMS chip 202 is mounted on and wire bonded to a substrate 206. The silicon microphone 200 may also include an optional logic chip 208. The micro-electromechanical chip 202 and the optional logic chip 208 may be connected by electrical leads. The silicon microphone 200 also has a lid 210 to cover the micro-electromechanical chip 202 and the optional logic chip 208. The lid 210 is usually electrically conductive for shielding purposes and is usually electrically coupled to a contact to be grounded.
The pads and contacts of the chips may be exposed and are susceptible to corrosion. While the logic chip may be covered with a polymer known as Globtop, covering the micro-electromechanical chip is not possible. Globtop is a specially formulated resin which may be deposited over a chip and its wire bonds, to provide mechanical support and exclude contaminants. Corrosion-resistant gold pads may thus be required for the micro-electromechanical chip. However, this may lead to increased costs.